This application relates to wireless communications systems, and more particularly to improved selection of networks in such systems.
When a user equipment (UE), such as a mobile phone or mobile station (MS), is registered on a visited public land mobile network (VPLMN), it sometimes periodically searches for higher prioritized PLMNs, as described in Section 4.4.3.3 of Third Generation Partnership Project (3GPP) Technical Specification (TS) 23.122, Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode, V6.4.0 (Rel. 6) and V7.1.0 (Rel. 7) (March 2005). It should be understood that citations of Release 6 and Release 7 specifications are not intended to exclude any other Releases.
U.S. Patent Application Publication No. US 2002/0119774 for “Method for PLMN Selection” by Johannesson et al. describes how a UE receives a list of data associated with networks neighboring the PLMN currently serving the MS from the base station (BS) of the PLMN currently serving the UE. A new PLMN to serve the UE can be selected based upon the list.
U.S. Patent Application Publication No. US 2004/0224689 for “Method for a Radiotelephone to Scan for Higher Priority Public Land Mobile Network” by Raghuram et al. describes how a radiotelephone can scan for available frequencies that are in use and supported by higher priority PLMNs and the radiotelephone.
The UE typically scans for higher prioritized PLMNs only if there are higher prioritized PLMNs in the same country as the VPLMN. In this process, the UE selects the highest prioritized available PLMN. For Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access (UTRA) Frequency Division Duplex (FDD) PLMNs, an “available PLMN” is a “PLMN for which the UE has found at least one cell and read its PLMN identity”, according to 3GPP TS 25.304, User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode, V5.8.0 (Rel. 5) and V6.5.0 (Rel. 6) (March 2005).
When the highest prioritized available PLMN has been selected, the UE leaves the current VPLMN and starts searching for a suitable cell belonging to this highest prioritized PLMN. One of the conditions for a suitable UTRA-network cell is fulfillment of its cell selection criterion S, which places requirements on received signal strength and quality. For UTRA, the cell selection criterion is defined in Section 5.2.3.1.2 of 3GPP TS 25.304, which provides that the cell selection criterion S is fulfilled when parameters Srxlev and Squal are greater than zero, for FDD cells, or when Srxlev is greater than zero, for time division duplex (TDD) cells. It will be understood that other cell selection criteria can be used for the methods and apparatus described in this application. It will also be understood that these methods and apparatus can be used for communication systems other than UTRA, for example as described in 3GPP TS 43.022, Functions related to Mobile Station (MS) in idle mode and group receive mode (Release 7), V7.1.0 (June 2005).
Srxlev is a cell selection receive level value (in dB) that is the difference between a parameter Qrxlevmeas and the sum of the parameters Qrxlevmin and Pcompensation. Qrxlevmeas is the measured cell receive level value, which is received signal code power (RSCP) on a common pilot channel (CPICH) for FDD cells (in dBm) and RSCP on a primary common control physical channel (P-CCPCH) for TDD cells (in dBm). Qrxlevmin is the minimum required receive level in the cell (in dBm). Pcompensation is max(UE_TXPWR_MAX_RACH−P_MAX, 0) in dB. UE_TXPWR_MAX_RACH is the maximum transmit power level a UE may use when accessing the cell on the random access channel (RACH) (read in system information) (in dBm), and P_MAX is the maximum radio frequency (RF) output power of the UE (in dBm). Of course it will be understood that other measurement units can be used for the parameters described in this application.
Squal is a cell selection quality value (in dB) for FDD cells that is the difference between the parameters Qqualmeas and Qqualmin. The parameter Qqualmeas is the measured cell quality value, i.e., the quality of the received signal expressed in CPICH Ec/N0 (in dB) for FDD cells. Qqualmin is the minimum required quality level in the cell (in dB) for FDD cells.
According to the above described behavior, the UE may select a PLMN for which no suitable cell exists. When this happens, the UE will continue the search for cells belonging to the selected PLMN on all supported frequencies and radio access technologies (RATs). A PLMN may support more than one radio access technology, and the UE determines what type of radio carrier to search for when attempting to select a specific PLMN (e.g., GSM, UMTS, GSM COMPACT, etc.). When this search is completed (often without finding a suitable cell), the UE will select another available PLMN, if found, and try to find a suitable cell on this other PLMN. A typical scenario is that the original VPLMN is then re-selected, and the UE starts looking for a suitable cell on the original VPLMN.
Among the consequences of this behavior is the UE will not be able to provide normal network services (e.g., incoming or outgoing voice calls) from the time the available PLMN was selected until the UE is back on the original cell on the VPLMN. In addition, the UE will spend significant energy searching for suitable cells and performing PLMN scans, without any benefit for the user or the network operators. Furthermore, the user will perceive worse than actual network coverage, since the UE will sometimes behave as if there is no coverage while in reality the VPLMN is available.